Re: Stabilizing the test_decoding checks, take N

Hi,

On 2022-04-16 13:11:59 -0400, Tom Lane wrote:
> My pet dinosaur prairiedog just failed in the contrib/test_decoding
> tests [1]:
>
> diff -U3 /Users/buildfarm/bf-data/HEAD/pgsql.build/contrib/test_decoding/expected/stream.out /Users/buildfarm/bf-data/HEAD/pgsql.build/contrib/test_decoding/results/stream.out
> --- /Users/buildfarm/bf-data/HEAD/pgsql.build/contrib/test_decoding/expected/stream.out 2022-04-15 07:59:17.000000000 -0400
> +++ /Users/buildfarm/bf-data/HEAD/pgsql.build/contrib/test_decoding/results/stream.out 2022-04-15 09:06:36.000000000 -0400
> @@ -77,10 +77,12 @@
> streaming change for transaction
> streaming change for transaction
> streaming change for transaction
> + closing a streamed block for transaction
> + opening a streamed block for transaction
> streaming change for transaction
> closing a streamed block for transaction
> committing streamed transaction
> -(13 rows)
> +(15 rows)
>
> Looking at the postmaster log, it's obvious where this extra transaction
> came from: auto-analyze ran on pg_type concurrently with the test step
> just before this one. That could only happen if the tests ran long enough
> for autovacuum_naptime to elapse, but prairiedog is a pretty slow machine.
> (And I hasten to point out that some other animals, such as those running
> valgrind or CLOBBER_CACHE_ALWAYS, are even slower.)
>
> We've seen this sort of problem before [2], and attempted to fix it [3]
> by making these tests ignore empty transactions. But of course
> auto-analyze's transaction wasn't empty, so that didn't help.

I don't quite understand this bit - the logic test_decoding uses to
decide if a transaction is "empty" is just whether a tuple was
output. And there shouldn't be any as part of auto-analyze, because we
don't decode catalog changes. I suspect there's something broken in the
streaming logic (potentially just in test_decoding) around
skip_empty_xacts.

> I think the most expedient way to prevent this type of failure is to run
> the test_decoding tests with autovacuum_naptime cranked up so far as to
> make it a non-issue, like maybe a day. Since test_decoding already adds
> some custom settings to postgresql.conf, this'll take just a one-line
> addition to test_decoding/logical.conf.

I'm a bit worried about this approach - we've IIRC had past bugs that
came only to light because of autovacuum starting. I wonder if we rather
should do the opposite and reduce naptime so it'll be seen on fast
machines, rather than very slow ones.

Greetings,

Andres Freund